Hydroxystyrenes, such as 4-hydroxystyrene (pHS) and acetylated derivatives thereof, such as 4-acetoxystyrene (pAS), are aromatic compounds that have potential utility in a wide variety of industrial applications. For example, these compounds have application in monomers for the production of resins, elastomers, adhesives, coatings, automotive finishes and inks, as well as in electronic materials. They may also be used as additives in elastomer and resin formulations.
A number of methods for the chemical synthesis of hydroxystyrenes and acetylated derivatives thereof are known. However, these methods require expensive reagents, harsh conditions, and give relatively low yields, typically between 30 to 63%. For example, Sovish (J. Org. Chem. 24:1345-1347 (1959)) describes a method for the preparation of 4-hydroxystyrene (also know as p-vinylphenol) from p-hydroxycinnamic acid (pHCA). The pHCA is decarboxylated in quinoline over copper powder at a temperature of about 225° C. The yield of 4-hydroxystyrene was about 41%.
Pittet et al. in U.S. Pat. No. 4,316,995 describe a method for preparing p-vinylphenol. In that method, p-hydroxybenzaldehyde is first reacted with malonic acid using ethylenediamine as catalyst to give pHCA, which is decarboxylated in situ at a temperature of 115 to 120° C. to form impure p-vinylphenol. The p-vinylphenol is isolated from the reaction mixture and reacted with acetic anhydride in the presence of a base, such as sodium hydroxide or potassium hydroxide, to form 4-acetoxystyrene, which is separated from the reaction mixture and hydrolyzed in the presence of a strong base to give purified p-vinylphenol. The yield of 4-vinylphenol was about 31%.
Schädeli in U.S. Pat. No. 5,274,060 describes a method for preparing 4-hydroxystyrene starting with pHCA. In that method, the pHCA is decarboxylated in dimethyl sulfoxide in the presence of an amine catalyst, i.e., 1,8-diazabicyclo[5,4-0]undec-7-ene, and hydroquinone at 135° C. to give 4-hydroxystyrene. The yield in that method was 63%.
Lala et al. in Australian Patent Application No. 7247129 describe a method for thermally decarboxylating ortho or para-hydroxyarylcarboxylic acids using an amine catalyst in an aprotic solvent to form the vinyl derivatives. Additionally, a method for preparing vinyl hydroxyaryl compounds by forming the hydroxyarylcarboxylic acid in situ, followed by thermal decarboxylation is described. The hydroxyarylcarboxylic acid is formed by reacting an aliphatic dicarboxylic acid or an aliphatic anhydride with a hydroxyarylaldehyde in a basic medium. The yields in those methods ranged from 15 to 60%.
Steinmann in U.S. Pat. No. 5,324,804 describes the synthesis of 3,4-dihydroxystyrene via the thermal decarboxylation of caffeic acid in dimethyl formamide at 150° C., in the absence of a catalyst. The yield obtained with that method was not given.
Munteanu et al. (J. Thermal Anal. 37:411-426 (1991)) describe the production of 3,5-di-tert-butyl-4-hydroxystyrene by the thermal decomposition of trans-3,5-di-tert-butyl-4-hydroxycinnamic acid with and without a non-amine basic catalyst in aprotic dipolar solvents. The reported yield was 95%. The thermal decarboxylation of other cinnamic acid derivatives is not described in the disclosure.
The thermal decarboxylation of substituted cinnamic acids has been studied in aqueous media. Pyysalo et al. (Lebensmittel-Wissenschraft u. Technol. 10 (Food Science and Technology):145-147 (1977)) describe the thermal decarboxylation of substituted cinnamic acid derivatives at pH 1 to 6 at 100° C. in aqueous buffer. Cohen et al. (J. Amer. Chem. Soc. 82:1907-1911 (1960)) describe the thermal decarboxylation of p-hydroxycinnamic acid in aqueous buffers at pH 1 to 12. The isolation of the decarboxylated product was not reported in those disclosures.
Therefore, the need exists for a method for preparing hydroxystyrenes and acetylated derivatives thereof that uses relatively inexpensive reagents, relatively mild conditions, and results in high yields.
Applicants have solved the stated problem by discovering a method for preparing hydroxystyrenes and acetylated derivatives thereof using relatively inexpensive reagents, under relatively mild conditions with yields up to 100%. The hydroxystyrenes are prepared by the thermal decarboxylation of a phenolic substrate in the presence of a non-amine basic catalyst. The acetylated derivatives are formed by reaction of the resulting hydroxystyrene with an acetylating agent in the same reaction vessel.